1. Field of the Invention
The present invention relates to a connecting head structure for high-pressure fuel injection pipes made from thick-wall steel pipes with a relatively small diameter disposed and commonly used as a supply passage for a fuel in a diesel internal-combustion engine, for example.
2. Description of the Related Art
A known conventional high-pressure fuel injection pipe having this type of connecting head has, as exemplified in FIG. 17, a connecting head portion 112 of a thick-wall steel pipe 111 with a relatively small diameter in which a spherical seat surface 113, an annular flange portion 115 provided with an interval from the seat surface 113 in the shaft center direction, and an arc surface 114 continued from the seat surface 113 to the annular flange portion 115 and tapered toward the tip end (See FIG. 4 of Japanese Patent Laid-Open Print No. 2003-336560). In this type of connecting head 112, in relation to molding by buckling machining by pressing with a punching member from outside in the shaft center direction, the inner diameter is expanded and stress is concentrated on the inner peripheral surface of the head portion with outward expansion of the peripheral wall used by the buckling machining by pressing so as to cause a pocket (annular recessed portion) 116 in which a tensile stress of the internal peripheral surface is increased and the connecting head has been used in this circumstance. However, there have been problems that cavitation erosion might occur in the vicinity of the pocket portion due to the high-pressure fluid when used, a radial crack might be generated radially in the connecting head portion from the pocket as a starting point due to fatigue breakdown, or a circumferential crack might be caused around the pocket due to the fatigue breakdown.
As a measure against those problems, the applicant proposed a method for making the pocket inside the head generated in molding of the connecting head shallow and gentle in a high-pressure fuel injection pipe having a connecting head of a thick-wall steel pipe with a relatively small diameter, for example, formed by a spherical seat surface, an annular flange portion provided with an interval from the seat surface in the shaft center direction, and a conical surface continued from the seat surface to the annular flange portion and tapered toward the tip end by providing an annular curved recessed groove with a small depth at a part of the conical surface (See FIG. 1 of Japanese Patent Laid-Open Print No. 2003-336560), a method for covering the pocket inside the head generated in molding of the connecting head having the outer peripheral surface in the truncated conical shaped or a truncated arc shaped seat surface with respect to the mating seat portion with a metal cylindrical member attached inside the head (Japanese Patent Laid-Open Print No. 2005-180218) and the like.
The present invention has an object to propose a connecting head structure for high-pressure fuel injection pipes which can obtain an effect equal to or better than the above proposed arts as means for preventing generation of a crack at a trough portion of the pocket involved with formation of the pocket in molding of the head portion, generation of a crack by cavitation erosion caused in the vicinity of the pocket portion by a flow of the high-pressure fluid when used, and generation of a phenomenon of tensile stress rise on the inner surface due to increased inner diameter and stress concentration due to formation of the pocket in molding of the head.